1. Technical Field
The present invention relates, generally, to machines for polishing or planarizing workpieces such as semiconductor wafers. More particularly, the present invention relates to a device which supports and engages a workpiece against a polishing pad surface.
2. Background Art and Technical Problems
Many electronic and computer-related products such as semiconductors, CD-ROMs, and computer hard disks, require highly polished surfaces in order to achieve optimum operational characteristics. Silicon workpieces or wafers are typically flat and circular in shape. For example, high-quality and extremely precise wafer surfaces are often needed during the production of semiconductor-based integrated circuits. During the fabrication process, the wafers generally undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures created on the wafer surface. As the size of integrated circuits decreases and the density of microstructures on integrated circuits increases, so too must the accuracy and precision of the wafer surface polishing also increase.
Chemical Mechanical Planarization ("CMP") machines have been developed to polish or planarize silicon wafer surfaces to the flat condition desired for manufacture of integrated circuit components and the like. For examples of conventional CMP processes and machines, see U.S. Pat. No. 4,805,348, issued in February 1989 to Arai et al.; U.S. Pat. No. 4,811,522, issued in March 1989 to Gill; U.S. Pat. No. 5,099,614, issued in March 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued in July 1994 to Karlsrud et al.; U.S. Pat. No. 5,476,890, issued in December 1995 to Masayoshi et al.; U.S. Pat. Nos. 5,498,196 and 5,498,199, both issued in March 1996 to Karlsrud et al.; U.S. Pat. No. 5,558,568, issued in September 1996 to Talieh et al; and U.S. Pat. No. 5,584,751, issued in December 1996 to Kobayashi et al.
Typically, a CMP machine includes a wafer carrier configured to hold and to rotate a wafer during the polishing or the planarizing of the wafer. The wafer carrier is rotated to cause relative lateral motion between the polishing pad and the wafer to produce a more uniform thickness. In general, the polishing surface includes a horizontal polishing pad that has an exposed abrasive surface of cerium oxide, aluminum oxide, fumed/precipitated silica, or other particulate abrasives. Commercially available polishing pads may utilize various materials, as is known in the art. Typically, polishing pads may be formed from a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Scottsdale, Ariz. The hardness and density of the polishing pad depends on the material that is to be polished and the degree of precision required in the polishing process.
During a polishing operation, a pressure plate, which forms the bottom of the wafer carrier, applies pressure to the wafer such that the wafer engages the polishing pad with a desired amount of pressure. The pressure plate and the polishing pad are also rotated, typically with differential velocities, to cause relative lateral motion between the polishing pad and the wafer to produce a more uniform thickness. The pressure applied through the wafer to the polishing pad causes the polishing pad to deform underneath the wafer surface, causing a `footprint`. As the wafer carrier moves across the polishing pad, the wafer footprint also moves with respect to the polishing pad. Therefore, elastic deformation and `spring-back` (swelling) of the polishing pad along the outer edge of the wafer continuously occurs during the polishing process. This effect is hereinafter referred to as `pad wave deformation`. The resulting non-uniformity of the polishing pad at the wafer edge causes an undesirable beveling of the wafer edge. Previously known methods for improving wafer flatness have addressed the wafer/polishing pad interface as a static footprint only, and thus these methods have not solved the problems resulting from the actual dynamic nature of the wafer footprint.
Prior attempts at reducing the effects of pad wave deformation include controlling the biasing pressure applied to the area outside the periphery of the wafer by the use of two separate fluid (air) pressure regulating mechanisms. Kobayashi et al. '751 teaches a first pressure regulating mechanism for controlling the biasing pressure applied to a wafer retaining ring and a second pressure regulating mechanism for controlling the pressure applied to the pressure plate. However, the use of two separate mechanisms to regulate air or other fluid pressure requires that the wafer polishing machine and each wafer carrier be provided with additional fluid lines, valves, and associated control equipment.
Therefore, an improved wafer carrier assembly and, in particular, a method for reducing the beveling effects of polishing pad wave deformation, is needed to address the above described limitations of the prior art.